Comparison between experimental and numerical results of a packed-bed thermal energy storage system in continuous operation

This paper reports the comparison between the results obtained by experimental tests made on a thermal energy storage (TES) system of sensible heat type under continuous operation with numerical simulations performed with a CFD model. TES system consists of a packed bed of spherical particles of alumina and air used as heat transfer fluid. The study concerns the time-dependent behaviour of the temperature field inside the bed, highlighting the thermocline evolution and its deterioration after repeated charge/discharge cycles due to the thermal hysteresis with a consequent progressive reduction of the recoverable energy. It is demonstrated that this effect is reduced by loosening the temperature constraints set at the end of each phase, increasing the amount of recovered energy and reducing the number of cycles needed to obtain the steady state. The CFD model was carried out considering a two-dimensional axisymmetric computational domain with the porous bed coupled to the wall. The governing equations are solved for incompressible viscous turbulent flow and fully developed forced convection based on the two-phase transient model equation to calculate both fluid and solid temperatures. Satisfactory agreement between detailed experimental and numerical results is reached when inlet air temperature follows that measured during experimental tests during both charge/discharge phases. CFD model used is reliable for predicting the whole temperature field of the TES during continuous operation. (C) 2018 The Authors. Published by Elsevier Ltd.